PHYSICS FOR THE IB DIPLOMA: TEACHER’S RESOURCE

Safety: If a real hammer is to be thrown, this should be done outside, preferably on a grass surface.

Assessment ideas: Students could put tracing paper over Figure 1.38 and trace out the parabola for themselves.

Differentiation: Students who need challenge can be asked why they think the centre of gravity, and not all parts of an object, follows a parabolic path.

Reflection: Ask students how easy or difficult they find visualising these concepts.

Language focus: Students may have used the term centre of gravity or centre of mass before. In practice, for small objects on Earth, they can be considered as being the same. The former is the point at which gravity acts and the latter is the point where the mass acts.

Plenary ideas

1 Falling balls (Time)

Resources: Two balls of approximately the same diameter but with different masses; a golf ball and a table tennis ball work well.

Description and purpose: Drop the balls at the same time from overhead, but do not stand on a desk or chair. Observe that the denser ball hits the ground first every time. Ask students to explain using ideas about air resistance why this happens. After all, the acceleration is the same for both, and air resistance should be the same because they have the same surface area and same shape.

Assessment ideas: This activity can be extended to a homework. If students have not met the equation F = ma, then this can be introduced as a hint.

Homework ideas

1 Practice questions

Homework should be differentiated according to need. For example, some students may need more practice at resolving vectors into components, while others may need practice at more complex projectile problems, such as objects projected at upward and downward angles off cliffs or tall buildings. Students should always be reminded of the symmetrical nature of the parabola, which may cut some steps from their calculations.

2 Falling balls (plenary extension)

If the Falling Balls plenary has been used, ask students to explain why the less dense ball reaches terminal speed first. Scaffolding can be provided as this is challenging. The force of air resistance is the same on both balls when their speed is the same, but because of F = ma, that force of air resistance has a greater decelerating effect on the ball with less mass.

CROSS-CURRICULAR LINKS

• The concepts of scalars and vectors arises again in many later topics, such as in Chapter 2, Forces and Newton’s Laws.

• An understanding of speed and velocity is required at many points in the course, such as with momentum in Chapter 4 and wave speed in Chapters 13 and 14.

• Gravitation and gravitational fields are studied in more depth in Chapter 17.